A common problem encountered in connection with power capacitors using dielectric oil containing PCB (polychlorinated biphenyl) compounds is bursting or rupture of the capacitor casing, particularly in the case of older capacitors utilizing paper insulation. Paper insulation can degrade gradually over an extended period, and gas evolved from decomposition of the paper and the dielectric oil can cause a dangerous pressure build-up within the capacitor casing. This can lead to swelling and ultimately rupture of the capacitor casing, resulting in power interruptions and requiring expensive cleanup measures.
Typically, such a capacitor is protected from bursting due to current overload by a fuse which blows at a high fault current. However, the build-up of pressure from paper degradation can result in rupture of the capacitor casing even in the absence of a current high enough to blow the fuse.
Pressure actuated switches are known to be used in induction heating devices, whereby a build-up of pressure over a selected limit will actuate the pressure actuated switch which in turn activates a disconnect relay to deactivate the device. Such a mechanism is relatively complex, and must be installed inside the device in order to be effective. Moreover, where the pressure buildup occurs from decomposition, merely deactivating the device will not prevent further buildup of pressure. In such a case indicating means is necessary to identify the problem before rupture of the device.